Grebes have strange feet, even by the standards of most birds. In fact, they are so strange that the grebe has abandoned the land and opted for an aquatic life, only leaving the water for floating nests to reproduce. Christoffer Johansson noticed their odd shape while wandering around the Natural History Museum in Gothenberg, and decided to set about answering the question: why do grebes have such weird feet?
His first problem was to find a trusty subject, so Johansson teamed up with Glufse the Grebe, and together they began to solve the problem of why grebe’s toes are the way they are.
Glufse was trained to swim through a 2 m long tunnel that was 0.4 m high and 0.6 m wide. Johansson filmed him from above and the side collecting over 170 swimming sequences. He rejected the more erratic trajectories, and finally selected the thirteen sequences where Glufse used his best technique for digital analysis. Johansson followed the motion of six points on Glufse’s body while he swam: the tip of his tail, two points on his leg and the tips of three toes. Having established the motions of the six body points during each swimming cycle, Johansson used force/vector analyses to draw the conclusion that grebes use lift, rather than drag, for forward propulsion.
How does Glufse achieve this feat? Well, his toes behave as hydrofoils. They act as multiple slots, which generate a lift force, and push the bird forward, in the way that primary flight feathers perform in birds. The multiple slots in the wings increase the lift to drag ratio by reducing the drag of the wings, and the gaps between Glufse's toes produce the same effect.
The asymmetric shape of the toes also reminded Johansson of primary flight feathers, which can be rotated individually to maintain the greatest lift coefficient. If this could be achieved by the toes swivelling passively, so that they were always in the best orientation to generate the least drag, this would maximise the lift to drag ratio and make this an extremely efficient way of swimming. Of course, Glufse does this with no effort at all.
Grebes have no living close relatives and probably haven’t evolved much over the last 2 million years. The fact that grebes evolved little over that period of time suggests that they have found an evolutionary pinnacle, but that doesn't mean that other swimming strokes aren't equally as efficient, just different.
So, that's why grebes have strangely shaped feet. And where is the star of Johansson's swimming movies? He has retired to his own private lake in the Universeum, Gothenberg, to paddle out peacefully the rest of his days.
